Blast at Thornton Quarry propels Deep Tunnel project

Work will create reservoir to hold billions of gallons of stormwater, sewer water overflow

September 24, 2013|By Andy Grimm, Chicago Tribune reporter

Onlookers gather as a blast sends tons of limestone flying Monday at the Thornton Quarry. A project for the Deep Tunnel floodwater control system will create the largest reservoir of its kind in the world to contain stormwater and sewer water overflow from the Chicago area. (Antonio Perez, Chicago Tribune)

A small crowd gathered Monday at the lip of the mammoth Thornton Quarry, all eyes fixed on an outcropping of dolomite nearly 300 feet below the shoulder of the westbound lanes of Interstate 80.

A ripple shot through the two-story rock formation, and it collapsed amid a small, dusty landslide. And so construction of the largest portion to date of the decades-in-the-making Deep Tunnel floodwater control system began with a bang.

"That was fun," said Metropolitan Water Reclamation District President Kathleen Therese Meany, smiling broadly as she turned away from the detonator box.

When it goes online in 2015, the Thornton Composite Reservoir will hold 7.9 billion gallons of stormwater and sanitary sewer water from more than a dozen south suburban towns.

The Thornton project will create the largest reservoir of its kind in the world and is the latest engineering marvel in nearly two centuries of trying to keep Chicagoland from reverting to its swampland roots while not turning Lake Michigan into a cesspool.

The blast also signals a shift in the landmark, 1,000-acre quarry that has been mined since 1837 and once was part of the industrial empire of Chicago's Crown family. The section of the quarry north of I-80 eventually will connect via a 30-foot-diameter tunnel to an 8-mile southerly section of the cavernous Deep Tunnel project. Mining will continue for decades in the larger portion of the quarry south of I-80, officials said.

By 2017, Thornton will lose the title of world's largest reservoir to another Deep Tunnel project, a 10 billion-gallon reservoir in McCook. All told, more than $35 billion has been spent on Deep Tunnel projects since work began in the late 1970s, and work won't be finished until at least 2029.

"This is one of the most visionary projects that has ever been done," said water district Executive Director David St. Pierre. He paraphrased legendary urban planner Daniel Burnham while commenting on the skepticism that has come with the Deep Tunnel system's high expectations and generational timeline.

"We live in a city of no small plans. This is certainly no small plan."

The 30-story-deep reservoir will fill like a regional bathtub during massive storms that threaten to overwhelm local sewer systems, a problem that has grown worse with more frequent and intense downpours in recent years and as development has replaced open, absorbent land with rooftops and pavement.

The Deep Tunnel project, a cavernous underground network of tunnels connecting about 350 square miles of storm sewers across the county, was first conceived in the 1960s. As it did back then, much of the region's stormwater travels through combined sewer systems that collect rainwater as well as wastewater for homes and industrial sites.

When those sewers become overfilled, they back up into basements or have to be emptied into streams and channels that feed into Lake Michigan, with a single storm sometimes forcing billions of gallons of bacteria-laden untreated sewage into the source of much of the region's drinking water.

Concerns about sewage entering Lake Michigan, and massive engineering feats intended to thwart such pollution, date back to the earliest days of Chicago history. Efforts to divert the flow of the Chicago River from the lake began as early as the mid-19th century, around the time the first quarries opened in Thornton.

The rock formation there is the remnant of a reef that flourished over much of the region during the Silurian period of 420 million years ago.

Blasts larger than the one Monday still shake the surrounding area as often as three times a day, said Marge Loitz, former president of the Thornton Historical Society. Thousands of motorists on Interstates 80 and 294 might not notice the explosions, but the quarry has been a regional curiosity. The historical society each year hosts two tours for up to 250 people each in cooperation with current quarry owner Hanson Material Service.

"We're booked until the summer of 2016 right now," Loitz said last week. "I don't know how people find out about (the tours). We sure don't advertise."

The sheer, clifflike sides of the reservoir will be visible almost in their entirety for all but a handful of days each year, when stormwater comes roiling in during a major rain, said Kevin Fitzpatrick, supervising engineer for the district's Thornton and McCook projects.

When a storm deluge threatens to back up municipal sewers, the three-story Deep Tunnel inlet at the base of the quarry's eastern edge will begin to fill the reservoir like a toilet bowl, albeit with dirty water.

The water will remain until it can be pumped back to treatment plants that currently are under construction, and then can be released into channels and streams. Finlike solar-powered aerators will float on top of the water, churning air into the stormwater to keep the site from becoming a smelly, 300-foot-deep swamp, Fitzpatrick said.